A very focused site would be easier to support and might be reasonable for hiring someone to do the scripting or learning how to do it yourself. You could stumble through the Amazon and other API's building on the sample scripts and merging the output together.
It would be easier to price check 20 stores for the prices of say "ham sandwiches" versus 20 stores for every product under the sun. You might even set up a system where it looks at the 20 stores for just the product codes you want, or product names.
However you might also try to build a community around "ham sandwiches" to draw people to your site. People might like to talk about upcoming brand availability, new concepts, how good they are for their child and such, well at the same time finding out where to buy them online.